1. Field of the Invention
The present invention relates to a distributed constant type electromagnetic delay line, and in particular to an improvement of such a type of electromagnetic delay line using a microstrip line and suitable for use in very high frequency ranges.
2. Description of the Prior Art
As an electromagnetic delay line of this kind there has been known a type using a microstrip line as shown in FIGS. 23 and 24 of the accompanying drawings. FIG. 23 is a partial perspective view of this electromagnetic delay line, and FIG. 24 is a partial sectional view thereof taken in a plane shown by the arrows XXIV--XXIV in FIG. 23. In this structure, the electroconductive strip is made by forming an electroconductive zigzag strip 5 which is bent back over a distance of W at intervals of P on one surface of a dielectric layer 3 whose other side is formed with a ground electrode 1.
An electromagnetic delay line of this structure is suitable for use in gigahertz bands and is adapted for compact design, but there is a limit to reduction in size and it has been difficult to achieve both super high speed and super compactness at the same time.
After studying the reasons for this, the inventor has discovered that the following are the factors which have prevented reduction in size.
In the above described electromagnetic delay line, a signal applied to the input end 7 is transmitted through the zigzag strip 5 in the direction indicated by the arrows. The zigzag strip 5 may be considered as being divided into a set of main electroconductive strips 9 which are disposed in parallel to the direction (the widthwise direction) perpendicular to the axial direction of the electromagnetic delay line, and a set of secondary electroconductive strips 11 which sequentially connect the main electroconductive strips 9 in series, and the electric current flowing through each of the main electroconductive strips 9 is opposite in direction to the electric current flowing through the one or two neighboring main electroconductive strips 9.
Considering the sectional view of FIG. 24, because the electric current flows in opposite direction as it flows through neighboring main electroconductive strips 9, the coupling coefficient between two immediately neighboring main electroconductive strips 9 has a negative value -k1, while two main electroconductive strips 9 separated by exactly one other main electroconductive strip 9 have a positive coupling coefficient k2 because the electric current flows in these two main electroconductive strips 9 take place in the same direction. Further, each certain main electroconductive strip 9 has coupling coefficients -k3, k4, . . . to other main electroconductive strips 9 which are displaced from that certain main electroconductive strip by two, three, . . . other intermediate electroconductive strips 9, because the electric current flowing through the main electroconductive strips 9 alternate in direction. As far as the absolute values of these coupling coefficients are concerned, they become smaller with an increase in the distance between the two main electroconductive strips 9 (K1 is greater than K2 is greater than K3 is greater than K4 . . . ).
And the effective inductance of each of the main electroconductive strips 9 is obtained by adding the effects of mutual induction determined by these coupling coefficients. Therefore, the effect of mutual induction is obtained by combination, and, taking into account the values and signs of these coupling coefficients, the combined value is always negative, and, to obtain the effective length of the zigzag strip 5 in this electromagnetic delay line, its actual length is to be reduced according to this combined negative value, and the delay time is accordingly reduced. Therefore, because it is necessary to compensate for the reduction in the effective strip length in order to obtain a desired delay time, size reduction of such an electromagnetic delay line is difficult. Furthermore, such an electromagnetic delay line having strong negative coupling not only tends to reduce the delay time but also gives rise to degradation of the flatness of the delay property and lowering of the cutoff frequency, thereby degrading the rise of the output pulse form.
In summary, electromagnetic delay lines using zigzag strips such as the shown zigzag strip 5 have been in existence in the industry in various forms, but there has been no electromagnetic delay line which has been analysed in terms of the values and the signs of various coupling coefficients, and none of the currently known types of electromagnetic delay line is sufficiently suitable for use in very high frequency ranges and very compact designs.